Sonic imaging systems have been utilized for biological and medical situations were interior points of a subject are to be observed. Typically, such imaging systems employ a source of sound in the form of an array of a relatively large number of small transducers or, alternatively, in the form of one or more large transducers having a size commensurate with the overall size of the array. The source of sound may provide for parallel rays of radiation to focus at infinity, or may provide converging rays to focus at a nearby point.
By way of example, the U.S. Pat. No. 3,967,234 which issued in the name of Jones on June 29, 1976, shows the use of arcuate transducers to provide converging beams of radiation for focusing at a nearby point to produce better resolution in the near field. A similar source of sound, but being composed of an array of ultrasonic transducers having a preset curvature for focusing the radiant energy, is shown in the U.S. Pat. No. 3,587,561 which issued in the name of Ziedonis on June 28, 1971.
A problem exists in the use of the foregoing systems for imaging a biological or medical subject in that, generally, the systems are only focused at one predetermined range from the source of sound. Attempts at variable focusing by use of a transducer array with a tapped delay line for each transducer have resulted in excessive complexity and cost for a commercially acceptable system. By way of example in a medical imaging system, such as the imaging of the internal portion of a living creature, it may be desirable to accurately discriminate between bone material at one depth and flesh material at a second depth. However, the use of the foregoing fixed-focus sound sources requires the selection of a predetermined depth which will be focused, while points of interest at other depths will not be as well resolved.